Interference Engine
An engine design where the pistons and valves occupy the same space in the combustion chamber at different times, risking catastrophic collision if the timing belt or chain fails.
An interference engine is an internal combustion engine design in which the valves open fully into the paths of the pistons. To maximize efficiency, compression, and airflow, engineers minimize the clearance volume in the combustion chamber. In normal operation, the timing belt or chain synchronizes the movements of the crankshaft and camshaft perfectly, ensuring that the pistons and valves never occupy the same space at the same time.
If the timing belt or timing chain snaps, stretches, or skips teeth while the engine is running, the camshaft stops spinning while the crankshaft continues to rotate. This causes the pistons to slam into the open, stationary valves at high speed. The result is catastrophic internal engine damage, including bent or broken valves, cracked pistons, damaged cylinder heads, and sometimes bent connecting rods.
Diagnosing interference engine damage typically involves a compression test or a borescope inspection of the cylinder chambers. If a timing belt fails on an interference engine, it usually requires a complete cylinder head rebuild or engine replacement, costing thousands of dollars. Drivers can prevent this by replacing their timing belt strictly at the manufacturer's recommended intervals, typically every 60,000 to 100,000 miles.